Microphones

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Resources and methods for learning about these subjects (list a few here, in preparation for yourresearch):

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Questions

Question 1

Explain how a moving-coil, or dynamic microphone works. Identify how the principle of electromagneticinduction is put to practical use in this device.

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Question 2

Certain substances are known to physically deform (shorten or lengthen) when an electrical voltage isapplied across their length. The effect is known as piezoelectricity. What types of substances are known tobe piezoelectric? Identify some possible applications of this phenomenon.

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Question 3

Explain how a crystal microphone works. Identify how the principle of piezoelectricity is put to practicaluse in this device.

Follow-up question: characterize the typical output impedance of this microphone type. Is it generallyconsidered a low-impedance or a high-impedance type of microphone? Why do you think this is?

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Question 4

The basic circuit for an unamplified capacitor microphone looks like this:

Vout

+V

Vibrations in the air cause one metal plate to vibrate next to a stationary metal plate, rapidly changingthe capacitance between the two plates. Explain how this simple circuit generates an AC voltage from thisrapidly-varying capacitance, employing the mathematical relationship between charge (Q), capacitance (C)and voltage (V ).

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Question 5

An older microphone technology is the carbon microphone. Explain how this type of microphone worked,and why it was so simple to use in a circuit.

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Answers

Answer 1

A moving-coil microphone is built just the way the name suggests: a lightweight coil of wire gets movedpast a magnet in proportion to vibrations in the air (sound), generating an AC voltage representative of thesound wave oscillations.

Follow-up question: characterize the typical output impedance of this microphone type. Is it generallyconsidered a low-impedance or a high-impedance type of microphone? Why do you think this is?

Answer 2

Certain crystal substances, such as quartz, are known to be piezoelectric.

Answer 3

In a crystal microphone, a small piezoelectric crystal is stressed and unstressed by sound wave pressure,generating a voltage representative of the sound wave oscillations.

Answer 4

Given the equation Q = CV , we can see that changes in capacitance will either result in changes ofstored charge for a given voltage (Q changes with V constant), changes in voltage for a given charge (Vchanges with Q constant), or changes in both Q and V . Remember that changes in electric charge over time(dQ

dt) is the mathematical definition of electric current (I).

Follow-up question: capacitor microphones are almost never used without some form of amplification.The voltage signal generated by the circuit shown in the question is simply too weak to use on its own.Explain why the signal is so weak.

Answer 5

A carbon microphone generates a changing resistance representing sound wave oscillations. Thesemicrophones could be used as easily as this, with no amplifier circuit necessary:

Carbonmicrophone

Moving-coilspeaker

Battery

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Notes

Notes 1

Ask your students where they located this information. it is important for students to learn where andhow to research for needed information.

Notes 2

As your students will no doubt discover in their research, piezoelectricity is a two-way effect: physicalmotion from electricity and visa-versa. It will be clear how well they did their research by the types ofapplications they think of for the ”electricity-to-motion” form of piezoelectricity, given the typical physicalscale of piezoelectric displacements.

Notes 3

Ask your students where they located this information. it is important for students to learn where andhow to research for needed information.

Notes 4

Ask your students where they located this information. it is important for students to learn where andhow to research for needed information.

Notes 5

Carbon microphones may represent an older, obsolete technology, but they are still illustrative forstudents.

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